LONDON, Jan 22 (Reuters) - Bank of England Governor Mervyn King is too powerful and planned changes at the bank will worsen the situation by the time Canada's Mark Carney succeeds him, a former senior policymaker at the bank said on Tuesday.

Adam Posen, speaking to legislators about three years on the bank's Monetary Policy Committee that ended in August, said the bank's non-executive directors, finance ministry officials and politicians were unable to effectively hold King to account.

British lawmakers have expressed concern about the Bank of England's accountability before, but Posen's comments are the strongest criticism so far from a recent MPC member.

King repeatedly opposed suggestions by external appointees to the committee to look at boosting the economy by buying large sums of assets other than British government bonds, said Posen.

This resistance in turn led to external MPC members being excluded from discussions early last year between Britain's finance ministry and the bank about a new Funding for Lending Scheme to boost bank credit, he continued.

"I was quite furious at the time, not at the governor - the governor was just advocating what he thought was right, frankly. My anger ... was at the people around Governor King and at the Treasury and in Court who let him get away with that," Posen said, referring to the BoE's supervisory Court of Directors.

New laws going through parliament are set to give the Bank of England's governor sweeping powers over financial regulation, something that raised further difficulties, Posen added.

"If it were up to me I would have written the statute differently ... because I frankly believe it goes too far," he said. "The governor of the Bank of England would be the most powerful single central banker in a major central bank.

CARNEY CHALLENGE

The practical ability of the Bank of England's governor to dominate the bank contrasts with the unusually wide scope that external MPC members have to publicly disagree with him - something Posen said might prove a challenge for King's successor, Mark Carney, who currently leads the Bank of Canada.

"I don't know about Mr Carney per se, but I think that anyone who is the governor of a central bank, where they're not a member of a committee structure and ... there aren't votes will have a very big culture shock," he said.

"Notwithstanding our discussion, there is more open debate ... in the Bank of England than in any other central bank that I am aware of.

The Bank of Canada does not have external appointees on its rate-setting board, and its officials tend not to publicly disagree with Carney, who starts at the BoE in July.

Posen also said two ideas associated with Carney - long-term monetary policy commitments, and targeting the cash size of the economy rather than inflation - would not be right for Britain.

"In my mind it would be a grievous mistake," he said about nominal GDP targeting, adding that it would push up inflation, which is already above the BoE's 2 percent target.

Interest rate commitments would be less damaging, but evidence suggested they did not work, he added.

Instead, the BoE needed to work with the finance ministry to go beyond the Funding for Lending Scheme to improve firms' access to capital markets, which is much more limited in Britain than in other developed economies, and even some emerging ones.

"I don't want to cause bad feeling, but I think the UK ... should be thinking the same way that Malaysia or Brazil five years ago or 10 years ago went to the IMF and asked 'what's best practice?' There are literally playbooks you can get," he said.

Posen also restated his view that finance minister George Osborne was cutting Britain's budget deficit too rapidly and that this was damaging economic confidence - something the central bank's political neutrality meant that he could not express while on the MPC.

Posen now heads the Petersen Institute, a leading U.S. economics think tank. (Additional reporting by Costas Pitas; editing by Patrick Graham)